1. Field of the Invention
This invention relates to a gas sensor adapted for detecting the concentration of a particular gas component in a gas to be measured such as an exhaust gas.
2. Description of the Related Art
A gas sensor installed in an exhaust system of an internal combustion engine has been known, which is used to detect oxygen concentration in exhaust gas for controlling combustion of the internal combustion engine. The oxygen sensor includes a sensor element that extends in an axial direction and is exposed to a gas to be measured at its front end; a tubular metal shell surrounding the sensor element; and a tubular protector that is fitted to the front end of the metal shell and which covers a front end portion of the sensor element protruding from the front end of the metal shell.
Metal parts that constitute the metal shell and the protector of the gas sensor are prepared from a metal material through a plurality of machining processes. During the plurality of machining processes, for instance, a plasticizing process, a cutting process and a grinding process, oil agents such as lubricants, cutting oils and grinding oils are used for lubrication and cooling.
The oil agent adhering to the metal is allowed to remain as is. This is because similar lubrication and cooling is desired in subsequent assembly processes (for instance, crimping of the metal shell and attachment of the protector and the metal shell) of the gas sensor. However, it is well known that when the residual oil agent is present in too much or too little an amount for a subsequent process, the residual oil agent should be partially washed away or more of the same oil agent should be applied so that the amount of adhering oil agent is suitable for the subsequent process. Furthermore, it is known that when the kind of residual oil agent of a processed article is not suitable for the subsequent process and another kind of oil agent is needed, all of the residual oil agent is washed away from the processed article, and an appropriate amount of a different oil agent is applied (JP-A-2005-193183, WO 2005/065846 A1).
If a gas sensor is assembled with the protector and metal shell on which an oil agent remains and then used in an exhaust pipe, the oil agent remaining on the inner peripheral surface of the protector and the inner peripheral surface of the metal shell is gasified. The resultant gas mingles with a gas to be measured to deteriorate the gas detection accuracy. To avoid this problem, JP-A-2005-193183 proposes a gas sensor, in which a metal shell and a protector are heated to gasify the oil agent adhering to the inner peripheral surface of the metal shell and the inner peripheral surface of the protector before the gas sensor is used so that the oil agent is removed in advance. However, in recent years, as emission regulations have become more rigorous, there is a need for higher gas detection accuracy. It is difficult to obtain a gas sensor that can sufficiently meet this need only by heating the gas sensor in advance. In particular, in recent gas sensors, the protector is being increasingly miniaturized. Even when a slight amount of oil agent remains on the inner peripheral surface of the metal shell and the inner peripheral surface of the protector, the gas sensor cannot provide the requisite high accuracy of gas detection due to the gasified oil agent.
The invention has been made to overcome the above-noted problems of the prior art, and an object thereof is to provide a gas sensor that can prevent deterioration of gas detection accuracy due to oil agent adhering to the inner peripheral surface of the protector and the inner peripheral surface of the metal shell.